The Adsorption and Surface Dilatational Rheology of Unmodified and

Chapter 18

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The Adsorption and Surface Dilatational Rheology of Unmodified and Hydrophobically Modified EHEC, Measured by Means of Axisymmetric Drop Shape Analysis Rolf

1,3

Myrvold ,

1,4

Finn K n u t Hansen , and B j ö r n L i n d m a n

2

1

2

Department of Chemistry, University of Oslo, P.O. Box 1033, Blindera, N-0315 Oslo, Norway Physical Chemistry 1, Chemical Center, University of Lund, P.O. Box 124, S-22100 Lund, Sweden

Studies of adsorbed and spread layers of ethylated hydroxyethyl cellulose (EHEC) and a hydrophobically modified analogue (HM-EHEC) at the air-water interface have been performed by means of four different experimental routes. The adsorption characteristics visualized by means of dynamic surface tension measurements, dilatational elastic behavior measured by means of a oscillating sessile bubble method, and the surface pressure response to surface area reductions measured by means of a Langmuir surface balance all give the same result. Relatively strong interactions can be observed when adding sodium dodecyl sulfate (SDS) to the polymer solutions. The SDS concentration for optimal enhancement of surface properties is 5-6 mM, indicating the formation of the most surface-active polymer/SDS complex. Despite the fact that the bulk properties of these two polymer/SDS systems are quite different, both the dynamic surface tension and the surface rheological properties are practically the same. This emphasizes the different effect of hydrophobic modification on bulk and surface behavior.

Introduction Several types of water-soluble polymers, and in particular biopolymers, play an important role in different types of applications. One class of water-soluble

Current address: Pronova Biopolymer, P.O. Box 494, N-3002 Drammen, Norway. Corresponding author.

2000 American Chemical Society

In Associative Polymers in Aqueous Media; Glass, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 2000.

303


304

biopolymers is made up of the different types of cellulose derivatives. These polymers may serve as stabilizers and as a part of the retard system in control drug delivery [1,2]. The equilibrium values of surface tension and adsorption are often used to describe the effect of surface-active materials or phenomena connected to colloidal stability. In many instances, these properties alone have been shown to be insufficient for correlation of the macroscopic behavior to fundamental microscopic phenomena, such as emulsification and foaming. In these systems it is often the dynamic properties of the interface that are important as a result of surface tension gradients and surface mobility. The Marangoni effect and the surface Theological parameters are used as a measure of the surface's properties under dynamic conditions. The surface shear viscosity and elasticity, the 2 dimensional equivalents to ordinary bulk rheology, have been shown to be closely correlated with the stability of emulsions and foams [27]. However, probably more important for these processes are the surface dilatational viscosity and elasticity that are often several orders of magnitude higher than the shear parameters [12]. These properties are, however, less readily measurable, except for the Gibbs elasticity that is the static limit of the latter. Several methods have been developed in order to measure the dynamics of adsorption and surface rheological properties, among these are the oscillating jet, oscillating bubble, surface waves etc. The interested reader is referred to ref. [12] for a further description of these methods and references. In this group we have developed an instrument for axisymmetric drop shape analysis [6] that can also measure dynamic properties of surface fdms by means of the oscillating bubble technique [7]. In this paper we will report the results from a study of the interfacial properties of ethylated hydroxyethyl cellulose (EHEC) and a hydrophobic modified analogue (HM-EHEC) and their synergies with sodium dodecyl sulfate (SDS). It has been shown that above a critical aggregation concentration (cac) of 4-5 mmolal SDS, which is below the critical micelle concentration for SDS, both of these polymers bind SDS in a cooperative manner [22,23]. Below this concentration, only HM-EHEC binds SDS non-cooperatively. It has also been shown that the bulk rheological properties of these polymers are greatly enhanced by SDS [8], but it has up to now been unclear to what degree the surface properties of EHEC are influenced by hydrophobic modifications and surfactant interaction. The adsorption characteristics of these systems are measured in this study by means of the axisymmetric drop shape analysis instrument, and the surface dilatational rheology is measured by means of the oscillating bubble method. A Langmuir surface balance is used for the measurements on the spread polymer layers [10].

Experimental Instruments The dynamic surface tension and surface rheological measurements were performed with an axisymmetric drop shape analysis instrument developed by this lab. This



305

instrument has been described elsewhere, [5-7], and will only be shortly reviewed here. All experiments were performed at 25 ± 0.5 °C. The instrument consists of a goniometer (Ramé-Hart) fitted with a macro lens and a CCD video camera. The video frames are captured by a DT3155framegrabber (Data Translation) in a Pentium PC. The drop is controlled by a dispenser and a specially designed oscillation unit consisting of a syringe with an excenter mounted piston that is motor driven. The dispenser is controlled by the PC. The dispenser and oscillation units are mounted in series with stainless steel pipes that are filled with distilled water. The drops and bubbles are extended from the tip of a small PTFE ('Teflon") tube into a cuvette inside a thermostatted and water filled chamber with glass windows. The Teflon tube contains an air pocket toward the water in the steel pipe. The specially written DROPimage computer program can control theframegrabber and the PC's RAM for picture storage, thus the program can capture pictures directly to RAM in real-time and calculate the results later. A maximum capture rate of 25 Hz in CCIR video is possible, and the rate of calculation is less than 0.2 s per picture (processor dependent). The program has many facilities for different measurement strategies, and is also able to keep the drop or bubble volume constant during a long period of time by means of a feedback method. This is very important when measuring adsorption in highly elastic films, where small deviations in volume (i.e. area) may cause significant measurement errors. The results that are calculated are the surface tension, shape factor (β), radius of curvature (Ro) the drop volume, height and width, the surface area, and the contact angle with the horizontal plane. A description of the instrument and the program may be found at http://www.uio.no/~fhansen/dropinst.html. An automatic Langmuir surface balance (Minitrough, KSV Instruments Ltd. Finland) was used to record the ΠΑ isotherms. The trough is madefroma single block PFTE and has a surface area of approximately 250 cm . The surface balance is kept in a glass cabinet placed on a stone table. The subphase temperature is controlled and maintained by water circulation, and was 25 °C throughout the study. The surface pressure of the monolayers is measured by means of a Wilhelmy plate, using a roughed platinum plate. 2

Chemicals The ethyl(hydroxyethyl) cellulose (EHEC) and the hydrophobically modified ethyl(hydroxyethyl) cellulose (HM-EHEC) were manufactured by Akzo Nobel A B , Stenungsund, Sweden and purified at the University of Lund. Both the unmodified and the hydrophobically modified polymers are ethyl(hydroxyethyl) cellulose ethers with the same molecular weight (Mw « 100.000 g mol" ). The average degrees of substitution of ethyl and hydroxyethyl groups are 0.6-0.7 and 1.8, respectively. These values correspond to the number of ethyl and hydroxyethyl groups per anhydroglucose unit of the polymer [8]. The HM-EHEC polymer is equivalent to the EHEC polymer sample, but with branched nonylphenol chains grafted to the cellulose backbone. The degree of nonylphenol substitution has been determined to be 1.7 mol % 1


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(approximately 6.5 groups per molecule) relative to the repeating units of the polymer [8]. Sodium dodecyl sulfate (SDS) of analytical grade was purchasedfromFluka and used without further purification. Sodium acetate and isopropanol of analytical grade were suppliedfromFluka and used without further purification. Water was de-ionized followed by distillation in an all glass still. The surface tension was measured to 72.3±0.1 mN m" at 21 °C and the conductance was measured to 1.8 cm" . The water was checked for surface-active impurities by compressing a 250 cm water surface on the Langmuir surface balance. Maximum compression gave surface pressures less than 0.1 mN m" . Polymer stock solutions (0.1 wt% (~1 g L" )) were prepared and allowed to equilibrate in a refrigerator for one month before further dilution. From these stock solutions new stock solutions of 0.01 wt% were made and equilibrated in refrigerator for seven days prior to use. The pure polymer and SDS/polymer solutions were made from these solutions by weighing a suitable amount of polymer and SDS and diluting this to the set concentration. This solution was let to equilibrate for at least two hours before use. 1


1

2

1

1

Procedure and data analysis The procedure has been described elsewhere [7] and only a short description will be given. A small amount of air (ca 150 uL) was sucked into the tip of the PTFE tube. The tip was dipped into ca. 10 mL of the polymer/SDS sample solution in the cuvette and fixed to the experimental chamber. After equilibration of a ca 10 pL bubble the computer was programmed to increase the bubble volume to 50 uL with immediate start of the surface tension measurement. The surface tension was measured as function of time for 50.000 s, until near equilibrium conditions were reached. In order to avoid compression effects due to bubble shrinkage the bubble volume was kept constant during the time of the experiment. After 50,000 s and completion of the dynamic surface tension measurement the drop was oscillated around the near equilibrium position with amplitude of ca 2.5 uL, corresponding to a surface area oscillation of approximately 1.5 mm . The oscillation was performed over a frequency interval between 0.2 and 2 Hz, and the presented rheological results are averages of six separatefrequencysweeps. 2

The calculation of the surface rheological parameters has been described elsewhere [7], but for completeness, as short description will be given here. For an oscillating bubble, we vary the surface area by changing the bubble volume in a sinusoidal manner with an angular rate co, and provided that the volume change is small, this results in a corresponding sinusoidal variation in the bubble surface area. This also leads to a corresponding surface tension variation. We can write this ΔΑ = A - A = A sin(cot) and 0

a

Δγ = γ - γ = Y sin(o)t + δ) 0

a


307

Here A is the area amplitude and Ao is the equilibrium surface area, y is the measured surface tension amplitude, γο is the equilibrium surface tension and δ is the phase angle. In the usual manner, the (complex) surface dilatational modulus is then expressed by A

a

E * = E'+iE"= |E|cos6 + i|E| sin δ

where

Y a

|E| =

A / Downloaded by STANFORD UNIV GREEN LIBR on September 22, 2012 | http://pubs.acs.org Publication Date: August 10, 2000 | doi: 10.1021/bk-2000-0765.ch018

a

AQ

Here E' is the storage modulus and E" the loss modulus. The moduli are connected to the Gibbs elasticity, Eo, as it is realized that Eo = dy/d(lnA) is the limit of E ' when ω —» 0 and E"=0. The contribution of the elastic and viscous terms depend on the different types of relaxation processes that occur in the surface layer and on the interaction of the surface with its surroundings, i.e. the bulk liquid(s). The loss modulus E" represents a combination of internal relaxation processes and relaxation due to transport of matter between the surface and the bulk. For the Langmuir surface balance measurements spreading solutions containing EHEC or HM-EHEC polymer were made according to a procedure developed for water soluble biopolymer systems [9], The technique used to spread the solution is described elsewhere [10]. After spreading of the polymers, enough time was always allowed for the surface pressure to reach a constant value. All glassware and PTFE equipment were washed with chromic sulfuric acid and rinsed with purified water prior to use.

Results and Discussion Due to their hydrophobic/hydrophilic nature water-soluble, cellulose derivatives are known to adsorb at the air-water interface. The adsorption of EHEC and HM-EHEC represented by the dynamic surface tension is shown in Figure 1. In this figure different mixtures of EHEC/SDS and HM-EHEC/SDS adsorbed to the interface are also included. In order to distinguish the short time behavior the time is plotted on a logarithmic scale. The adsorption of surfactants and macromolecules to the air-water interface is well described in the literature [2-4, 11-14]. Adsorption of macromolecules to the airwater interface is believed to be characterized by three distinct regimes [2-4]: 1.

2. 3.

The lagtime regime where the surface tension is more or less constant. This regime is characterized by diffusion of macromolecules from the solution to the interfacial region where it is adsorbed. A regime where the surface tension drops relatively fast. This regime is characterized by spreading and/or unfolding of the adsorbed macromolecule. The regime where the rate of surface tension reduction decreases towards steady state/equilibrium conditions. In this regime molecular rearrangements to optimize the surface energy are the dominating process.



308

Figure 1 shows that these characteristic regimes can also be seen for the adsorption of EHEC and HM-EHEC to the air-water interface. The adsorption dynamics of similar types of EHEC- and other cellulose derivatives is well described elsewhere [2-4, 11] and will therefor not be treated in detail here. What can be seen is that despite the hydrophobic modification in HM-EHEC the adsorption dynamics of this compound is very similar to the EHEC adsorption. It is known that EHEC is a polymer of mixed hydrophobic/hydrophilic nature. What is not known is the distribution of hydrophobic/hydrophilic segments along the polymer chain. Since the HM-EHEC molecule contains on average 6.5 hydrophobic groups more than the EHEC molecule, it is natural, on the basis of these measurements, to assume that the hydrophobic modifications in HM-EHEC are connected to the already hydrophobic segments or parts of the polymer backbone and by this changing the overall surface activity of the polymer very little.

Time [s] Figure 1. Surface tension measured as function of time for EHEC/SDS and HMEHEC/SDS systems. Polymer concentration 10 ppm (wAv), and CMC = 8 mM for SDS.fFor the sake of clarity only 50% of the data points are shown.)

When increasing the SDS concentration and keeping the polymer concentration constant the lagtime is shifted down towards lower surface tensions. From Figure 1 it can be seen that as for the pure polymer-water systems there is no significant difference between the unmodified polymer and its modified analogue. Further the



309

figure shows that even for high SDS concentrations the lagtime remains and can be measured. During the lagtime period the surface tension should be the same or very close to that of the solvent. Since the SDS adsorption is fast, too fast to be measured by this experiment, the surface tension during this period should reflect the equilibrium surface tension of a solution of that particular SDS concentration. This means that what we really measure is the adsorption of EHEC/HM-EHEC polymer into an already present SDS layer. After approximately 10-100 seconds the lagtime period ends and the adsorption process enters the regime where the surface tension decreases rapidly. Figure 1 shows that in this regime the rate of surface tension reduction depends very much on the SDS concentration in the solution. This is probably connected to the solution properties (thermodynamic conditions) for the polymer molecule in the two-dimensional interface region. To illustrate this consider a spread polymer monolayer for which scaling theory states that in the semi-dilute regime the surface pressure depends on the surface concentration according to the power law [16-19]: Π~Γ*~Α*

(1)

Here Π = γο - γ is the surface pressure defined as the lowering in surface tension due to the presence of thefilm-formingmaterial (relative to the surface tension of the pure solvent, γο), Γ is the surface concentration and A is the mean surface area occupied by one molecule. The exponent y is a measure of the thermodynamic conditions in the surface system and can be expressed as: y = dv/(dv-l)

(2)

where d is the space dimensionality, in this case 2, and ν is a characteristic exponent to express the concentration dependency of the radius of gyration (an excluded volume/area scaling exponent). For polymer chains in two dimensions ν is calculated to be 0.77 for good solvents and 0.505 for theta solvents [20]. From Equation (2) it can be seen that good thermodynamic conditions will be characterized by a smaller slope (in log-log scale) than for systems where the thermodynamics approaches theta conditions. Despite the fact that this scaling theory is not developed for dynamic systems, this may serve as an illustration of the differences in dynamic surface tension reduction since the surface concentration also will be a function of time. The differences in thermodynamic conditions may be caused by the presence of the SDS surfactant or is a representation of the adsorption/formation of a more surface-active polymer/SDS complex [4, 21]. Figure 2 shows the surface pressure area isotherms measured by means of the Langmuir surface balance of EHEC and HM-EHEC spread on solutions with different SDS concentration (the surface pressure is measured with respect to the surface pressure of the pure SDS subphase). On water the amount of polymer spread is barely enough to give a measurable surface pressure on compression of the surface. Spreading of the same amount of polymer on subphases with increasing SDS concentration gives dramatic effects.



310

Figure 2 shows that in the same way as for the dynamic surface tension results there are only small differences between the spread layers of the two polymers. Further it can be seen that the surface pressure increases, that the shape of the surface pressure isotherm becomes more expanded, and that the area per molecule is shifted towards higher values at constant surface pressure, as the SDS concentration increases. This effect is observed up to a certain SDS concentration range only. Considering Equation (1) this effect is expected since the interactions between the polymer and the interface are expected to become more favorable as the SDS concentration is increased. This is expected to happen up to a certain level of SDS and beyond this the solubility of the polymer/SDS complex in the solution increases. In Figure 3 is shown the surface pressures at molecular area of 15000 Â as function of the SDS concentration in the subphase. From Figure 3 it is clear that the surface activity of the polymer/SDS system reaches a maximum value in a certain SDS concentration range. The development in surface pressure at constant area per molecule (constant polymer surface concentration) illustrates the above discussion. The polymer interacts in the interface with the SDS molecules to form more surface-active complexes. This process continues up to a certain SDS level and beyond 5-6 mM SDS the surface activity decreases and is probably caused by enhanced solubility of the polymer/SDS complex in the subphase. Above the CMC for SDS (8 mM) the polymer is solubilized by the surfactant and the surface pressure of the polymer becomes very low. Figure 1 shows that after ca 200-1000 seconds the adsorption process enters the third and final regime where the surface tension reduction is mainly governed by molecular rearrangements/internal relaxation processes. Because of the apparent linear relationship in the double logarithmic plot, the decrease in surface tension can be described by to the equation 2

tfO-f

(3)

where the exponent, a, will be a characteristic parameter for the internal relaxation in the layer. The power law described by Equ. 3 may also be derived theoretically by assuming diffusion controlled adsorption in the presence of a kinetic barrier (10). The exponent, a, is then proportional to the activation energy. Figure 4 shows the power law exponent calculated from the data presented in Figure 1 as function of SDS concentration. The exponents were calculated for times larger than 2000 s. The figure shows how the exponent varies with the SDS concentration. The rate reaches a maximum at SDS concentrations around 5 mM and is approximately the same for EHEC as for HM-EHEC. This increase in the rate of surface tension decrease is probably associated with the growth of a polymer/SDS complex as already mentioned, and the maximum value represents an optimum composition for the most surface-active complex. Addition of SDS beyond 5 mM increases strongly the amount of SDS bound to the bulk polymer, which leads to an increased solubility and thus a lower surface activity of the complex. At 9.9 mM SDS the dynamic surface tension of the system at these long times is close to that of a pure SDS system. These features are also seen in viscosity measurements of EHEC/surfactant systems in solution [22,23].



311

15

30

45

Ο

15

30 2

4

Area per molecule [Â ! (χΙΟ ) Figure 2. Excess surface pressure/area isotherms of EHEC and HM-EHEC monolayers spread on SDS solution with different concentration. (For the sake of clarity only 11% of the data points are shown.)

Ε z

=4= 'SDSl'

Figure 3. The surface pressure of EHEC and HM-EHEC at a molecular area of 15000 A fromfigure2, is plotted as function of the SDS concentration in the subphase. The molecular area corresponds to a surface concentration ofca 0.11 mg m . 2



312

C

S D S

[mM]

Figure 4. The long time power law exponent, ou presented as function of the SDS concentration.

From Figure 1 it can also be seen that in this regime some of the surface tension curves drop below the curve of 9.9 mM SDS. This may also serve as an illustration of the presence of a polymer/SDS complex that is more surface-active than the separate compounds. Even after 50,000 seconds equilibrium or steady state is not reached, and one can speculate on whether the low SDS concentration systems also after long enough times will drop below the 9.9 mM SDS system. After spreading of 0.8 pg polymer on different SDS solutions, the growth in surface pressure as function of time can be measured with a Langmuir surface balance. These results are reported in Figure 5. The figure clearly shows how the rate of surface pressure increase is affected by different SDS concentrations. Surfactant concentrations from 1 mM up to 5 mM cause an increase in the surface pressure growth rate, and also in the plateau value that is approached after more than 2000 seconds. Increasing the SDS concentration further beyond 5 mM causes a reduction in the surface pressure growth rate, with declining plateau values. HM-EHEC spread on 9 and 10 mM SDS hardly shows any increase in surface pressure relative to the system without SDS. These observations are in line with the previous interpretations of enhanced surface activity due to the presence of a more surface-active complex. The presence of polymer/surfactant complexes should indeed be revealed through surface rheology measurements. It is expected that a more surface-active complex should give a higher response in the elastic behavior than the case is for a less surface active compound [21]. Figure 6 shows thefrequencydependency of the elastic and


313


viscous moduli of adsorbed EHEC/SDS and HM-EHEC/SDS layers and shows what already was mentioned.

Time [s] Figure 5. The increase in surface pressure after 0.8pg of EHEC or HM-EHEC has been spread on subphases with different SDS concentration. Surface concentration ca 0.11 mg ni . 2

Figure 6 also shows that the elastic moduli, E \ are much higher than the corresponding viscous moduli, E " which are close to zero. This is a consequence of a very low phase angle, which reflects the elastic nature of the interfacial layers and the lack of transport between the bulk and the surface in the time scale of an oscillation period. Within the experimental errors the viscous moduli (±1.6 mN m" on a 95% confidence limit level [7]) all extrapolate to approximately zero as the frequency vanishes. This reflects the insoluble nature (irreversibly adsorbed) of these layers since a higher value at low frequencies can be interpreted as a result of desorption [12,25]. The figure also shows that the elastic moduli increase with frequency, a behavior that is characteristic for insoluble monolayers/irreversibly adsorbed layers [12]. From the figure one can also see that within this frequency regime there are no major differences between the EHEC/SDS and HM-EHEC systems except that the elastic modulus for the EHEC/SDS systems is slightly higher in the 4-5 mM SDS interval. For both systems the level of the elastic moduli is shifted higher as the SDS concentration increases. This behavior is observed up to a certain SDS concentration. Beyond this SDS concentration the elastic modulus levels off and declines towards 1


314


lower values again. This behavior is summarized in Figure 7 where the elastic moduli at 0.6 and 1.0 Hz are presented as function of SDS concentration for the two polymer systems.

ω

1

[rad s" ]

Figure 6. Surface elastic and viscous moduli measured as function of frequency for EHEC/SDS and HM-EHEC/SDS systems.

The increase in elasticity observed in Figure 7 represents in addition to the presence of a more surface active polymer/SDS complex the ability the surface layer has to restore surface tension uniformity upon deformations. This ability is a property of the greatest importance for foaming and emulsification processes [24]. Through this figure it can also be observed that there is a small difference between the EHEC/SDS and HM-EHEC/SDS systems when the location of the optimum composition for enhanced surface elasticity is considered. The EHEC/SDS system reaches the optimum value in surface elasticity at a lower SDS level than for the corresponding HM-EHEC/SDS system. If this effect is significant (and not an artifact) it may be caused by the more hydrophobic HM-EHEC and this requires more SDS in order to obtain the optimal surface-active complex (optimal hydrophobic/hydrophilic balance). The absolute value of the elastic modulus is also higher for EHEC than H M EHEC, and can probably be explained in the same manner.


315


τ—·—ι—•—ι—«—ι—·—ι—'—ι—•—ι—·—ι

ρ

0-h—ι—ι—ι—ι—ι—ι—ι—ι—ι—ι—·—ι— """1—•—I—' 0 1 2 3 4 5 6 7 8 9

1

I

1

Γ]

f 10

CsDstmM] Figure 7. Surface elastic moduli at 0.6 Hz (solid symbols) and 1.0 Hz (open symbols for EHEC (circles) and HM-EHEC (squares) adsorbed layers as function of SDS concentration.

Conclusion The results from dynamic and static experiments performed on adsorbed and spread EHEC/SDS and HM-EHEC/SDS layers have shown that the interfacial behavior can be designed from the composition of the system. Enhanced surface properties, such as dynamic surface tension, dilatational elastic modulus and the surface pressure serve to show that special synergism between these two polymers and the surfactant SDS takes place. The formation of what is believed to be a more surface-active polymer/SDS complex can be observed through four independent types of experiments. The SDS concentration that give the optimal composition for the formation of the most surface active polymer/SDS complex is determined to be 5-6 mM and was observed in all four experiments. This concentration coincides quite well with the cac of the parent polymer system (EHEC). Above the cac both polymers bind SDS in a cooperative manner [22], which makes the polymer more soluble and thus less surface active. Below the cac there is definitely a polymer-surfactant interaction that materializes in a surface pressure and elasticity increase. Only small or no differences between the EHEC/SDS and the HM-EHEC/SDS systems could be observed. This is in strong contrast to bulk behavior of these polymer/surfactant systems, where a strong effect of the hydrophobic modification is observed [8]. The non-cooperative binding of SDS to HM-EHEC polymer thus does not seem to result in enhanced surface activity. The rationale for this difference is probably connected to


316

the fact that the hydrophobic modification is relatively small, and probably grafted to the polymer backbone in regions of an already hydrophobic character. Because EHEC is considerably surface active in its unmodified form, this modification is not sufficient to alter the overall surface activity of the polymer.


References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27.

Pharmaceutical Technology, Yearbook 1998, 32. Persson, B., Nilsson, S. and Sunderlöf, L.-O. Carbohydrate Polymers 1996, 29, 119. Ybert, C. and di Meglio, J.-M. Langmuir 1998, 14, 471. Nahringbauer, I. Langmuir 1997, 13, 2242. Hansen, F.K. and Rødsrud, G. J. Colloid Interface Sci. 1991, 141, 1. Hansen, F.K. J. Colloid Interface Sci. 1993, 160, 209. Myrvold, R. and Hansen, F.K. J. Colloid Interface Sci. 1998, 207, 97. Thuresson, K., Nyström, B., Wang, G. and Lindman, B. Langmuir 1995, 11, 3730. Ställberg, S. and Teorell, T. Trans. Faraday Soc. 1939, 35, 1413. Myrvold, R., Hansen, F.K., and Balinov,B. Colloids and Surfaces A 1996, 117, 27. Nahringbauer, I. Progr. Colloid Polym. Sci. 1991, 84, 200. Dukhin, S.S., Kretzschmar, G. and Miller, R. Dynamics of Adsorption at Liquid Interfaces, Studies in Interfacial Science; Elsevier: Amsterdam, 1995. Tsay, R-Y., Lin, S-Y., Lin, L-W. and Chen, S-I. Langmuir 1997, 13, 3191. Hansen, F. K. and Myrvold, R. J. Colloid Interface Sci. 1995, 176, 408. Boury, F., Ivanova, T. Z. Panaïotov, I., Prost, J. E., Bois, A. and Richou, J. J. Colloid Interface Sci. 1995, 169, 380. Jiang, Q. and Chiew, Y. C. Macromolecules 1994, 27, 32. Vilanove, R. and Rondelez. F., Phys. Rev. Lett. 1980, 45, 1502. Vilanove, R., Poupinet, D. and Rondelez, F. Macromolecules 1988, 21, 2880. Kawaguchi, M., Komatsu, S., Matsuzumi, M . and Takahashi, A. J. Colloid Interface Sci. 1984, 102, 356. Takahashi, Α., Yoshida, A. and Kawaguchi, M . Macromolecules 1982, 15, 1196. Regismond, S. T. Α., Gracie, K. D., Winnik, F. M . and Goddard, E. D. Langmuir 1997, 13, 5558. Thuresson, K., Söderman, O., Hansson, P., Wang, G. J. Phys. Chem. 1996, 100, 4909. Thuresson, K., Lindman, B. J. Phys. Chem. B 1997, 101, 6460. Lucassen, J. and van den Tempel, M . Chem. Eng. Sci. 1972, 27, 1283. Lucassen-Reynders, E. H. Surfactant Science Series 1991, 11, 173. Myrvold, R., Hansen, F. K., Balinov, B. and Skurtveit, R. J. Colloid Interface Sci. 1999, 215, in press. Edwards, D. Α., Brenner, H. and Wasan, D. T. Interfacial Transport Processes and Rheology; Butterworths-Heinemann Publishers, 1991.


The Adsorption and Surface Dilatational Rheology of Unmodified and

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